Hi,
You can change it through UCI commands.
Type uci show | grep sim and you should see plenty of options to change.
We're looking for something like this: simcard.@sim[0].primary='1'
If we don't want sim[0] (SIM 1) to be primary, we will have to delete this line in UCI and create it onto sim[1] (SIM2).
uci delete simcard.@sim[0].primary='1'
uci add simcard.@sim[1].primary='1'
uci commit
reload_config
Regards.